In recent years, there has been a demand to reduce tire rolling resistance from the perspective of fuel efficiency when a vehicle is traveling. In addition, there has been a demand for enhancement in wet performance (control performance on a wet road surface) from the perspective of safety. To meet these demands, a method of establishing compatibility for low rolling resistance and wet performance by compounding silica to a rubber component constituting a tread portion of a tire is known.
However, silica has low affinity with rubber components, and the cohesiveness between silica components is high, so if silica is simply compounded to the rubber component, the silica is not dispersed, which leads to a problem that the effect of reducing the rolling resistance or the effect of improving the wet performance cannot be sufficiently achieved.
Therefore, International Patent Application Publication No. WO 2014/050341 discloses conjugated diene-based rubber obtained by reacting a conjugated diene-based polymer chain having an active terminal with a predetermined modifying agent. International Patent Application Publication No. WO 2014/050341 describes that by using the above conjugated diene-based rubber, the affinity between the silica and the rubber is good, and reduced heat build-up (low rolling resistance) of a tire and wet grip performance can be enhanced.
Meanwhile, environmental issues and resource problems have led to a demand for a higher fuel efficiency in vehicles, which in turn has led to a demand for further enhancements in the low rolling resistance of tires. In addition, along with improvements in the required safety level, there has also been a demand for further improvements in wet performance. Further, from the perspective of a production process, there has also been a demand for reduction of viscosity and improvement in workability.
To reflect on these, when the present inventor, et al., studied the conjugated diene-based rubber described in International Patent Application Publication No. WO 2014/050341, it became obvious that the rubber composition for a tire obtained did not necessarily satisfy the level recently required for the viscosity, the workability, the wet performance, and the low rolling resistance when formed into a tire.